MICROBIOLOGY: No Coughing in the Cattle Shed

Tradition has it that Mycobacterium tuberculosis, the causative agent of tuberculosis, evolved from M. bovis, which displays a broad host spectrum and is thought to have colonized human beings as our ancestors domesticated cattle. Brosch et al. used deletion analysis to locate 20 variable regions found among the otherwise very similar (99.9%) genomes of the tubercle group bacilli. They discovered that the polymorphisms have not evolved independently but appear to have resulted from a few ancient events in ancestral strains. A corollary finding is that these regions can serve as diagnostic markers for different strains; one in particular, called TbD1, is specific to M. tuberculosis and discriminates between ancient and modern pandemic strains of the pathogen, suggesting that the ancestral strain was African or Asian. Progressive mapping of the deletions indicated that M. bovis evolved from M. tuberculosis and that its genome is smaller than that of M. tuberculosis. Previously, Cole et al. had shown that considerable gene decay has occurred in a related, specialist human pathogen, M. leprae, which since its divergence from their last common ancestor appears to have lost 2000 genes. – CA